The present invention relates to photomultiplier tubes and, more particularly, to a photomultiplier tube having a conductive strip disposed on the envelope sidewall which is connected to a photoemissive cathode. Electrical connection to the conductive strip is provided by a resilient cathode contact member which is attached to a cathode contact support member. The cathode contact support member includes alignment means which aligns the resilient cathode contact member with the conductive strip. The tube includes a shield cup which has attached thereto an electron multiplier cage assembly. The shield cup is centered by means of a plurality of bulb contacts which are disposed within the bulb locating means formed in the shield cup. The bulb locating means are oriented such that the bulb contacts can contact the sidewall of the tube envelope without contacting the conductive strip connected to the photocathode.
In U.S. Pat. No. 4,370,585, issued to G. N. Butterwick on Jan. 25, 1983, shows a photomultiplier tube having an evacuated glass bulb with a shield affixed to one end of a dynode assembly and having a plurality of prong-like bulb contact members attached to the outer surface of the shield. The first dynode of the dynode assembly is affixed to the shield. An aluminum coating is disposed on the sidewall of the bulb, and a portion of the coating extends longitudinally along the sidewall and is contacted by a tab-type cathode contact member which comprises one element of an evaporator structure affixed to one side of the dynode assembly. A photoemissive cathode is formed on an interior surface of the bulb and on a portion of the aluminum coating. In the structure described in the foregoing Butterwick patent, the shield operates at a potential different from that applied to the cathode; thus, it is necessary that the bulb contact members, which center the dynode assembly, do not contact the aluminum coating on the envelope. If the bulb contact members are improperly positioned on the shield so that they contact the aluminum coating, an electrical short results between the cathode and the first dynode of the dynode assembly which is attached to the shield.
U.S. Pat. No. 4,426, 596, issued to G. N. Butterwick on Jan. 17, 1984, discloses a photomultiplier tube comprising a glass bulb having a plurality of circumferentially disposed strip-like bulb spacers attached to the outer surface of the shield cup. The shield cup supports a dynode cage assembly having a first dynode which is electrically connected to the shield cup. An aluminum coating is disposed around the faceplate of the glass bulb and in a longitudinally extending strip along a portion of the sidewall of the bulb. A photoemissive cathode is formed on the interior surface of the bulb and on a portion of the aluminum coating. A tab-like cathode contact is attached to a stem lead to apply cathode potential to the aluminum strip. The bulb spacers must be attached to the outer surface of the shield cup in such a manner that when inserted into the glass bulb, the bulb spacers do not contact the aluminum strip, or the shield cup and the first dynode will be shorted to the cathode rendering the tube inoperable.
The requirement that the bulb spacers do not contact the aluminum strip on the bulb sidewall requires careful fabrication and inspection of the tube before sealing to prevent loss of tubes due to electrical shorts. In order to improve efficiency and reliability of the tube, it is desirable that the tube structure provide means for accurately positioning the bulb spacers relative to the aluminum strip to prevent contact therebetween. It also is imperative that the cathode contact is accurately located with respect to the aluminum strip on the sidewall of the bulb to ensure that a potential can be applied to the strip and, thus, to the cathode.